The identification of transmission path characteristics is not only important for assuring reliable transmission over the transmission path, but also, as is the case for an optical link, assuring the safety of people using the optical link. The safety issue occurs when the optical transmitter transmitting on the optical link uses a laser. Since light from a laser can damage a person's eyes, precautions must be taken to prevent the laser from being turned on if the optical transmitter is not properly terminated by the optical link.
In most transmission systems, losses and delays are introduced into the transmission path by the transmission media, splices, connectors, splitters, combiners, amplifiers, repeaters, attenuators, etc. This is particularly true of fiber optic transmission systems. The result is that receivers have to be more complex and costly in order to compensate for optical loss and delay in various optical paths. If the losses vary greatly from receiver to receiver due to a varying number of passive and active optical devices within the transmission path, the receivers are not capable of compensating for this variation. In this case, each individual receiver must be adjusted for the signal level and delay.
One prior art method for correcting this type of a situation is to manually adjust the receivers by physically adjusting the receiver or by entering information into a computer controlling the transmission system and having the computer adjust each individual receiver. The problems with a manual adjustment procedure are expense and probability of human errors.
Where the attenuation is the only concern, U.S. Pat. No. 5,060,302 discloses an optical receiver feeding back information to the optical transmitter to adjust the output of the optical transmitter. There are two problems with this prior art solution. First, it only functions where a transmitter is driving a single receiver; and second, it requires an additional optical transmitter and receiver for the feedback path which is expensive.
Another prior art method which does not require manual entry of data is disclosed in U.S. Pat. No. 4,295,043. This patent discloses the use of a connector which identifies the length of attached cable by predefined electrical contacts placed on the connector of the cable. Different connectors are used for different lengths of optical fiber when the cable is being assembled. The receiver then automatically adjusts to the cable length based on the electrical contacts and assumes a predefined transmitter output. This method does allow a receiver to adjust for particular lengths of optical fiber and a given transmitter output. However, it does not allow two lengths of optical fiber cable to interconnect the transmitter and receiver. Nor does the method allow for any type of passive or active optical devices to be in the communication path from the transmitter to the optical receiver.
There exists a need for a method which allows a plurality of optical receivers to automatically adjust to the signal levels communicated to these receivers from a single transmitter when the optical transmission paths to each of the receivers is different due to the introduction of passive and active optical devices. The need for this solution becomes more acute as optical fiber links are utilized in the office and residential environments. As optical systems are utilized in either of these environments, it becomes necessary to introduce a variety of passive and active devices between a single transmitter and a plurality of receivers. The expense and the probability of error of manually adjusting each receiver becomes prohibitive in these environments.
Returning to the safety problem, the prior art has used two methods to assure safety when a laser is driving an optical link. The first method is to use mechanical interlocks to assure that an optical link is connected to a transmitter before the laser can be turned on. The problem with this method is the expense of providing the interlocks. Further, light transmitted from a laser via a multimode optical fiber can still damage a person's eyes so that the mechanical interlocks can only be used with single mode optical fiber. U.S. Pat. No. 5,039,194 discloses the second method that uses an optical transmitter and receiver at each end of the optical link. Each transmitter transmits a very short pulse (which will not cause eye damage), and the associated receiver waits to detect a pulse from the other transmitter at the other end of the optical link. If both receivers receive the pulses, the transmitters begin normal operations. This method is very expense since it requires a transmitter and receiver at both ends of the optical link. Also. the costs of the control circuitry is high. As lasers gain wider use in office and residential environments, the safety problem will become more important. Hence, there exists a need for a cheaper and more reliable way to assure safety when a laser is used to drive an optical link.
Further, there exists a need in large optical transmission systems to verify the actual optical components against the planned optical components in any given optical path within the optical transmission system. This need is particularly relevant to the residential environment which requires a large number of optical components.